H+ Pitch Angle Distributions in the Outer Magnetosphere Observed by MMS HPCA

Abstract

AbstractWe analyze 3 years of 1 eV–40 keV H+ pitch angle distributions (PADs) near the magnetic equatorial plane, observed by the Magnetospheric Multiscale (MMS) Hot Plasma Composition Analyzer (HPCA). Our analysis includes an energy correction to shift ion energy upward by an amount equal to the spacecraft potential. We find asymmetric distributions for cold (<10 eV) and warm (10–100 eV) H+ ions coming preferentially from the northern hemisphere (NH) over those from the southern hemisphere (SH). Specifically, the average phase space density (PSD) in the antiparallel direction (pitch angle above 135°) is about 1.7 times higher than in the parallel direction (below 45°) for H+ with energy <100 eV on the dawn side (MLT: 3–9 hr) for all L shells. To understand the cause of this NH‐SH asymmetry, we sort the distributions by Interplanetary Magnetic Field (IMF) direction and dipole tilt angle. Sorting by IMF direction shows that PSDs in the antiparallel direction are higher than in the parallel direction when IMF By is dawnward (By < 0). The outflow of cold H+ from the NH is higher when the dipole tilt angle is positive (μ > 10°). In contrast, when the dipole tilt is negative (μ < −10°), the outflow of cold H+ from the SH is not significantly higher and PADs can have either symmetric or asymmetric favoring from NH. These results suggest there may be general interhemispheric asymmetry of ion outflow that favors the NH.